Leftist runner-up Andres Manuel Lopez Obrador, who lost the presidential election by 3.3 million votes, challenged the result, alleging that Pena Nieto's party used slush funds to buy votes and breached spending limits.

Lopez Obrador demanded that the electoral court void the result and the suit has left Mexico in political limbo for weeks, denying Pena Nieto a head start in building consensus on economic reforms in Congress, where his centrist Institutional Revolutionary Party (PRI) failed to win enough seats for an outright majority.

Electoral officials told Reuters the ruling is due by Thursday or Friday, although the court has until September 6 to decide on Lopez Obrador's charges. Officials have told Reuters privately they do not expect Pena Nieto's win to be overturned.

The court said in a statement it had distributed a draft ruling to its seven member judges, but gave no details.

Lopez Obrador has accused the PRI of buying some 5 million votes, and has publicly presented receipts and tax documents to allege that the PRI used shell companies to fund the fraud.

Pena Nieto, who is due to take office on December 1, has rejected the claims, which election experts say are nearly impossible to prove unless someone is caught in the act.

Showing that a party handed out gifts does not prove that they coerced voters into casting a ballot for them, they noted. Parties can distribute gifts as part of their propaganda.

Legal experts are adamant that Lopez Obrador, who also unsuccessfully challenged the result of the 2006 presidential election, which he lost by less than one percentage point, has failed to prove that irregularities occurred on a big enough scale to justify throwing out the result.

"The issue is if it altered the general will, and what we have here are isolated cases," said Jose Vargas, the former top prosecutor for electoral crimes in Mexico.

TURNOUT BOOST

The accusations revived memories of vote-rigging and corruption when the PRI ruled Mexico between 1929 and 2000. The PRI won a return to power partly thanks to higher support among the poor, who critics say are a soft target for vote-buying.

In the poorest towns of Chiapas, Mexico's most impoverished and southernmost state, the PRI's coalition partner, the Green Party, was accused of orchestrating the drive to buy votes.

In the poor mountain village of Santiago el Pinar, the Greens won the local elections held on July 1 even though they had never received a single vote there before.

"The night before the election, they came in offering people money. That is how they won," said Lorenzo Gomez, a local resident.

The Greens deny the accusations.

Turnout was unusually strong in Chiapas during the election, surging 18 percentage points from the 2006 contest. In Santiago el Pinar, 93 percent of registered locals took part in the vote, about 30 points higher than the nationwide figure.

Chiapas was among the states Lopez Obrador said had suffered the worst voter fraud by the PRI and its allies. But experts say he has not presented enough evidence to make his case stick.

"The evidence is really weak," said Luis Ugalde, who led the Federal Electoral Institute in 2006. He, like many analysts, say Lopez Obrador is trying to distract from the loss and defend his position as the leader of Mexico's political left.

LOW QUALITY DEMOCRACY

Still, analysts agree that the allegations underscore Mexico's slow political development, where few officials are convicted, or even face charges, in corruption scandals.

"We have a low-quality democracy," Ugalde said. "Political impunity continues to be the same as before."

President Felipe Calderon's National Action Party (PAN) has also said there were signs the PRI used suspect funds and bought votes, but it has not demanded that judges invalidate the election. Instead, they are calling for electoral reform.

Luis Alberto Villareal, the PAN leader in Mexico's lower house, said that his party would push legislation that would make breaking campaign spending limits grounds for annulling an election.

Both Lopez Obrador and the PAN have alleged the PRI spent between 4 billion and 5 billion pesos (around $300 million to $380 million) -- or about 13 times the legal limit -- on the presidential campaign.

Parties are often accused of securing funds via illegal private donations and siphoning funds from local governments. While the federal government has improved transparency, state and local finances remain opaque and corruption is widespread.